BP saw nearly £8bn wiped off its value yesterday after US President Barack Obama said the company must pay the clean-up costs of the giant oil spill in the Gulf of Mexico.

The oil major's share price plunged another 6 per cent as the US military stepped in to help contain the 105-mile slick heading rapidly for the Louisiana coastline.

BP's stock has dropped by a dizzying 11 per cent since the explosion on the Deepwater Horizon rig that claimed 11 lives last Tuesday night. And even this week's financial results recording a £4bn doubling of its profits will not stop the sell-off.

The company says it is doing everything it can to help shut down the remains of the well, which has been pouring more than 200,000 gallons of oil into the sea every day since subcontractor Transocean's burnt-out rig sank on Saturday.

BP has 69 boats of various types working around the clock to contain the spill. So far, it has put down more than 100,000ft of floating barriers, sprayed more than 76,000 gallons of dispersant and skimmed off 685,000 gallons of contaminated water.

The company has also had five remote-controlled robots labouring 5,000ft down on the sea floor since Sunday, in a desperate effort to close the "blow-out preventer" mechanism which should have shut the well off automatically in the first place. In case the robots fail, BP engineers are also designing a cap that can be fixed to the top of the well – although the scheme could take as much as two weeks to complete.

"We are attacking this spill on all fronts, bringing into play all and any resources and advanced technologies we believe can help," Tony Hayward, BP's chief executive, said yesterday. "The scale of the surface response is truly unprecedented."